A difficulty is found in the measurement of weak electromagnetic radiation, such as optical radiation or electromagnetic radiation of wavelengths of about 0.01 to 1000 micrometers (um). Such weak radiation may comprise the light produced by the emission of a fluorescent or luminescent specimen or the reflectance signal of a particle in a suspension media as in the measure of a turbid specimen for the determination of concentration.
In such circumstances, a difficulty is in the discrimination between the inherent noise of a detection system and the signal value produced by the specimen. The Signal to Noise Ratio (SNR) is a measure of signal value relative to the noise value of a detection system. In practice, a signal value approximately twice the noise value is considered a practical limit of a detection system for discriminating with confidence the signal value from the noise value.
Various methods can be employed to reduce inherent noise value of a detection system, such as cooling of the detector to reduce the thermal generation of random electrons or by employing signal processing techniques such as signal averaging. But these methods are limited in application or effectiveness, wherein a limit is reached wherein little or no further reduction of the noise value can be achieved by signal processing.
If the noise value of the detector cannot be further reduced, then improvements to the SNR can only be achieved through increases in the signal value. One common method employed to improve the signal value is to concentrate the radiation onto the detector. Another method is to increase the intensity of the stimulating radiation. However, increasing the intensity of the stimulating beam may result in damage of the specimen due to heating or breaking of molecular bonds, and is therefore limited to some threshold of practicality.
Conventional optical elements for the concentration of optical radiation, such as lenses or mirrors, are directional in nature, collecting radiation emitted along a specific ray path from a specific direction or area of origin. Typically, systems utilized in the detection of weak optical signals are positioned substantially perpendicularly to the incident beam of stimulating radiation so as to maximize the SNR. Additionally, much of the radiation emitted by fluorescence or particle scatter goes undetected as consequence of a finite subtended angle of the radiation concentrator device.
Another source of noise which can affect the SNR of an optical measurement system is stray radiation. Stray radiation is detectable radiation which impinges upon the detection device, generating a signal unrelated to the specimen or electromagnetic phenomenon under examination. As an example, radiation which is received in the detector, but which did not propagate through or interact with the sample, is a common stray radiation.